The MaxDiskUsage directive configures the maximum amount of
disk space in use, as a percentage, before uploads to the server (i.e.APPEs, STORs, and STOUs) are denied.
Denial of these commands will be logged.

The percent can also be "none", which is used to override
any inherited disk use limits, as from a <Global> context.

The optional parameters are used to restrict the disk usage limit only to
specific users. If "user" restriction is given, then
expression is a user-expression specifying to which users the limit
applies. Similarly for the "group" restriction. For the
"class" restriction, the expression is simply the name of connection
class for whom the rule will apply.

Example:

# Prevent anonymous users from using up too much space...
MaxDiskUsage 60.0 user anonymous
# ...but allow other users to use up to a more reasonable amount
MaxDiskUsage 80.0